The present invention relates to a positioning device which utilizes radio waves from transmission sources, a positioning control method, and a recording medium.
A positioning system has been used in practice which locates the present position of a GPS receiver utilizing a satellite navigation system such as a global positioning system (GPS).
The GPS receiver receives a clear and acquisition or coarse and access (C/A) code which is one type of pseudo-random noise code (hereinafter called “PN code”) carried on a radio wave from a GPS satellite (hereinafter called “satellite radio wave”) based on a navigation message indicating the orbit of the GPS satellite and the like (including almanac (approximate satellite orbital information), ephemeris (precise satellite orbital information), and the like). The C/A code is a code which forms the basis for positioning.
The GPS receiver determines the GPS satellite which has transmitted the received C/A code, and calculates the distance (pseudo-range) between the GPS satellite and the GPS receiver based on the phase of the C/A code (code phase), for example. The GPS receiver locates the position of the GPS receiver based on the pseudo-range between the GPS receiver and each of three or more GPS satellites and the position of each GPS satellite in the satellite orbit. For example, the C/A code has a bit rate of 1.023 Mbps and a code length of 1023 chips. Therefore, it is considered that the C/A codes line up in units of about 300 kilometers (km) over which a radio wave advances in 1 millisecond (ms). Therefore, the pseudo-range can be calculated by calculating the number of C/A codes between the GPS satellite and the GPS receiver from the position of the GPS satellite in the satellite orbit and the approximate position of the GPS receiver. In more detail, the pseudo-range can be calculated by calculating one cycle (1023 chips) of the C/A code (integer portion of the C/A code), and specifying the phase of the C/A code (fraction portion of the C/A code). The integer portion of the C/A code can be estimated when the approximate position of the GPS receiver has a specific accuracy (e.g. 150 km or less). Therefore, the GPS receiver can calculate the pseudo-range by specifying the phase of the C/A code.
The GPS receiver correlates the received C/A code with a C/A code replica generated by the GPS receiver, accumulates the correlation values, and specifies the phase of the C/A code when the correlation cumulative value reaches a specific level. The GPS receiver performs the correlation process while changing the phase and the frequency of the C/A code replica.
However, a sufficient signal strength cannot be obtained when the radio wave strength of the satellite radio wave carrying the C/A code is weak, whereby it becomes difficult to specify the phase of the C/A code.
In order to deal with this problem, technology has been proposed which coherently (synchronously) and successively combines the process results of segments of a received signal until a threshold signal-to-noise ratio (SNR) is achieved (e.g. JP-T-2004-501352).
However, since the GPS satellite and the GPS receiver move relatively, the arrival frequency of the satellite radio wave which reaches the GPS receiver changes due to the Doppler shift.
When the signal strength is weak, it may be difficult to synchronize the synchronization frequency of the GPS receiver with the arrival frequency which continuously changes.
When the synchronization frequency of the GPS receiver differs from the arrival frequency, the accuracy of the phase of the C/A code deteriorates even if the correlation cumulative value reaches a specific level. Therefore, the accuracy of the located position may deteriorate when locating the position using the above phase.
Accordingly, the present invention may provide a positioning device, a positioning control method, and a recording medium storing a positioning control program capable of verifying the accuracy of the phase of the positioning base code and accurately locating the position under a weak electric field in which the radio wave strength is weak.